1. Technical Field
Embodiments relate to the field of integrated circuits. More especially, embodiments relate to integrated circuits comprising isolating trenches that each delimit an active area made of a semiconductor material as well as the corresponding method for making them.
2. Discussion of the Related Art
The production of MOS transistors with a high-permittivity dielectric gate is known. Integrating a layer configured to adjust the work function of a gate into a gate of such a MOS transistor is also known.
FIGS. 1, 1AA and 1BB are, respectively, a top view, a cross-sectional view along plane AA in FIG. 1 and a cross-sectional view along plane BB in FIG. 1 of a MOS transistor 1 produced on and in a portion of a semiconductor substrate 2.
Substrate 2 comprises an isolating trench 4, commonly referred to as Shallow Trench Isolation (STI) that delimits an active area 6 made of monocrystalline silicon. On the front face 7 of substrate 2, there is a gate 8 that is isolated from the substrate by an insulating interface layer 10 made of silica. Gate 8 successively comprises a high-permittivity dielectric layer 12 made of hafnium oxide placed on layer 10, a lanthanum or magnesium layer 14 adapted in order to adjust the work function of the gate, a metal layer 16 made of titanium nitride and, finally, layer 18 made of polycrystalline silicon. Two spacers 20 are located on either side of gate 8.
In an effort to simplify the drawings, the source and drain areas of transistor 1 are not shown.
Once layer 14 has been formed, the lanthanum present in this layer 14 tends to diffuse during each of the subsequent stages that requires a certain thermal budget. The Applicant has, in particular, noted the occurrence of this diffusion phenomenon at a temperature of approximately 500° C. The lanthanum has a tendency to be absorbed by the oxygen reservoirs and to form compounds of the La2O3 or lanthanum silicate type. The concentration of lanthanum in layer 14 diminishes in an uncontrolled manner. This layer is therefore unable to continue fulfilling its function: this falsifies adjustment of the work function of the gate. Because the threshold voltage of a MOS transistor is proportional, in particular, to the work function of its gate, the operating characteristics of the transistor are thus modified and are no longer controlled.
The same applies if layer 14 comprises magnesium rather than lanthanum. The magnesium diffuses and seeks out oxygen reservoirs in order to form MgO type compounds.
The Applicant has also noticed this same affinity with oxygen when the lanthanum and magnesium are deposited respectively in the form of La2O3 and MgO.